Patent GB-635695-A (GB)
Title: Improvements in or relating to feedback amplifiers
IPC: H03F1/36
Filed: 1946-11-26
Abstract: 635,695. Valve amplifying circuits. DUERDOTH, W. T. Nov. 26, 1946, Nos. 34976/46 and 16347/47. [Class 40 (vi)] In a feedback amplifier having at least two negative feedback paths each applying a voltage to an input circuit of the amplifier, the voltage applied to one feedback loop is derived from a point in the forward path of the amplifier so that the feedback voltage is unaffected by stray valve capacities, the ultimate slope of the gain/frequency characteristic of the loop being lower than that of another feedback loop so that the ultimate slope of the gain/ frequency characteristic of the combined feedback loops beyond either or both ends of the working band of the amplifier is decreased and in this way the amplifier is rendered stable or the margin of its stability is increased. Fig. 6 shows an amplifier having a main negative feedback path from the output transformer T2 via the feedback transformer T3 to the input transformer T1 which has the gain/ frequency characteristic shown in Fig. 7, Curve 1. The margin of stability is improved at frequencies above and below the working band, i.e. the ultimate slope of the gain/frequency curve is reduced, by the subsidiary feedback networks Zl, Z2 in the cathode circuit of the valve which have gain/frequency characteristics as shown in Fig. 7, curves 2, 3 respectively, the combined feedback networks having the characteristic shown in Fig. 7, curve 4. The stability of the amplifier may be maintained in spite of variations in its terminating impedance, the ageing of valves, and the overloading of particular stages, by terminating the amplifier in a high and low-pass filter pair. Since the termination of the amplifier is of particular importance at those points in the frequency range at which the magnitudes of the main and subsidiary feedback voltages are comparable, i.e. at frequencies outside the working range, the cut-off frequencies of the filters are arranged to lie between these points and the end frequencies of the working band. The termination within this critical range is an impedance, preferably a resistor, of the same value as the normal termination of the amplifier. To prevent the overloading of the output stage of an amplifier with the consequent reduction of its gain, from causing instability, an earlier stage is arranged to overload at a level above that necessary to produce overloading of the outputstage but below that which would render the amplifier conditionally stable or unstable. Fig. 10 shows a multi-stage amplifier terminated in the low-pass filter comprised by network L3, L5, C3, C4 and the high-pass filter L4, L8, C1, C2, R1, working into a load L. The amplifier has a main feedback path from the output transformer T2 via network F to network G in the input circuit and two subsidiary paths (a) from the anode of the phase-splitter stage V2 to the cathode of valve V1 via the network J and thus to the input circuit, and (b) that constituted by the network shunted across the resistor 10 in the cathode of valve VI. The gain/frequency characteristic of the feedback paths of this amplifier are plotted in Fig. 11 (not shown). The anode current of valve V2 is adjusted so that the output valves V3, V4 overload before V2 and when V3, V4 overload the reduction in their input impedance due to grid current flow causes V2 to overload immediately. The Nyquist diagram of this amplifier using pentode valves is plotted in Fig. 13 (not shown). The Nyquist diagram does not enclose the point 1/0, i.e. the amplifier remains stable even if the output circuit of the amplifier is open or short-circuited, Fig. 14 (not shown). Specification 514,567 is referred to.